1. Field of the Invention
The present invention relates to apparatus for supervising and controlling the output power of induction motors and, more particularly, to circuit means which is capable of checking the loading conditions of an induction motor to a higher precision in a range exceeding the rated maximum load and which is capable of indicating rotary speed in terms of a percentage of synchronous speed.
2. Description of the Prior Art
In an induction motor, as the load and the output torque increase, the rotary speed decreases and simultaneously the electric current consumption increases. It is therefore possible to supervise the loading conditions of an induction motor by measuring and supervising the rotary speed. However, since the change in the rotary speed with respect to the load variation is small, it is difficult to satisfactorily supervise changes in the rotary speed by use of a conventional tachometer.
Synchronous speed (S) is calculated from the line frequency (f) and the number of poles of a motor (P) by the following equation, which is independent of the output capacity: EQU S=(120 f)/(P) . (1)
The slip (S1) of an induction motor is proportional to the difference between the synchronous speed and the actual speed as follows: EQU S1=(S-Ha)/(S).times.100% and (2) EQU S1max=(S-H)/(S).times.100%, (3)
where Ha=the actual motor speed in operation and H=the speed at the rated maximum output.
For most induction motors, slip at rated maximum ranges from 0.3 to 5%. Due to the very small value of slip, only a very narrow portion of the scale of a tachometer is useful as an effective reading range and precision of the scale reading is very poor. For example, if one is to determine the output of a 4-pole motor, the maximum slip of which is 5% and which is operated at a line frequency of 50 Hz, the rotary speed of the motor varies from 1500 to 1425 rpm as the load changes from 0-100% of the rated maximum. Assuming that the tachometer has a scale of 0 to 1500 rpm divided by 100, misreading of one division causes only a 1% error when it is used as a tachometer but when the tachometer is used as an output meter, the same misreading causes a 20% error.
An apparatus is also known for supervising the loading conditions by supervising the electric current consumption. This, too, is not satisfactory as the change in the electric current consumption with respect to the change in load is too small.
Induction motors are also used in applications where load variations are greater than usual, such as motors for agitating devices or for machine tools. In monitoring the load of an induction motor involving a load greater than the rated maximum, no sufficient supervision was heretofore possible by simply checking the rotary speed or the electric current consumption.
In my copending U.S. patent application Ser. No. 59,261, filed July 20, 1979, entitled APPARATUS FOR THE MEASUREMENT OF THE MECHANICAL OUTPUT OF INDUCTION MOTORS now U.S. Pat. No. 4,281,288, there is disclosed circuit means for providing a quick and easy measurement of the output power of an induction motor. The apparatus comprises means for generating a first voltage signal (S) proportional to the synchronous speed of the motor, means for generating a second voltage signal (Ha) proportional to the actual speed of the motor in operation, means for generating a third voltage signal (H) proportional to the speed of the motor at the rated maximum output, and means for generating a voltage signal proportional to S-Ha and indicating such signal in terms of a percentage of the rated maximum. The signal proportional to S-Ha is applied to a meter for providing a continuous indication of the output power of the motor.
While the apparatus for my copending application is exceedingly effective for measuring the output of an induction motor while the motor is operating within the range of the rated maximum output, the output thereof under overload conditions can not be accurately measured. That is, for an accurate measurement of the output of an overloaded motor, it is necessary to suspend the measurement in order to readjust the established value of the slip and the full scale of the meter. For example, by setting a 6% slip for an induction motor having a maximum slip of 4%, measurement of the output under an overload of up to 150% becomes possible. It is also possible to measure the output under an overload of 125% or 200% by readjusting the calibration of the meter so as to point the full scale at 80% or 50% of the original full scale reading.
However, overloading conditions appear only for a very short period of time in the actual operation of an induction motor and it is not appropriate to measure the output under overload by the method described above. The apparatus of my prior application is inconvenient in that the meter is incapable of directly indicating the speed of rotation and is constructed to merely indicate the output, from which the speed of rotation at the time of the output has to be calculated from an equation.